Stethoscope

ABSTRACT

The stethoscope is characterized by a housing having an external body contact membrane. A pickup plate is mounted in the housing near the membrane. An electromagnetic coil and armature are mounted on a support plate in the housing. The armature is mounted on the side of support plate opposite to the pickup plate and is connected to the pickup plate so that vibrations detected by the pickup plate are transmitted to the armature. In this way, when the body contact membrane is pressed against the body of a person, with sufficient force to contact the skin clothing interface, the armature is forced away from the electromagnetic coil so it can vibrate freely. With this arrangement the maximum sensitivity of the transducer coincides with the optimum acoustical matching between the body and the transducer.

United States Patent 2] Inventor Donald Rowley 3,311,703 3/1967 Grinstead 179/1(ST) 16042 entu a Woodland Hills, 3,396,241 8/1968 Anderson et al 179/1(ST) Cahf' 91364 Primary Examiner-Kath1een H. Claffy [21] Appl' 613361 AssistantExaminerDou lasW Olms 22 Filed Dec. 19, 1966 g [45] Patented Jan. 12,1971

ABSTRACT: The stethoscope is characterized by a housing having an external body contact membrane. A pickup plate is [54] g gggg gzg Figs mounted in the housing near the membrane. An electromagnetic coil and armature are mounted on a support plate in the Cl 179/1 housing. The armature is mounted on the side of support plate [51] Int. Cl H041- 1/46, opposite to the pickup plate and is connected to the pickup 6 7/ plate so that vibrations detected by the pickup plate are trans- [50] Field of Search 179/1, 1ST in d to the armature. In this way, when the body contact membrane is pressed against the body of a person, with suffi- [56] References cued cient force to compact the skin-clothing interface. the arma- UNITED STATES PATENTS ture is forced away from the electromagnetic coil so it can 2,755,336 7/1956 Zener et al 179/1(ST) vibrate freely. With this arrangement the maximum sensi- 3,187,098 6/1965 Farrar et a1. 179/1(ST) tivity ofthe transducer coincides with the optimum acoustical 3,233,041 2/1966 Croslin 179/1(ST) matching between the body and the transducer.

i 5 -'-l8 35 222 20 f g as 52 34 as srrsrrroscorn "this invention relates generally to a stethoscope and more particularly to an electronic stethoscope.

Stethoscopes are used by physicians in mediate auscultaiion. By means of the stethoscope the physician is able to study sounds produced in the heart, lungs, stomach,'and other portions of the body. Of these the most important are heart sounds, murmurs, breathing sounds, respiratory rales or rattles and perlastic squeaks or groans. The stethoscope is used to enable the physician to more clearly hear the body sounds because diagnosis is based in part on the structure of the sounds the physician can hear.

The sounds of the body range from about 40 cycles to 4000 cycles. In particular, the fundamental frequently of the systolic heart sounds range from 40 cycles to 80 cycles with overtones up to around 4000 cycles. The fundamental diastolic heart sounds range from 60 cycles to 100 cycles with overtones ranging up to 4000 cycles. The fundamental sounds of systolic and diastolic murmurs range from 300 to 800 cycles. The fundamental sounds of prestolic murmurs range from 60 to 200 cycles. It is apparent therefore that to be useful, a stethoscope should have a good response from 40 to 4000 cycles. In addition, because the sound frequencies of interest may vary, it is desirable for the stethoscope to have means for emphasizing the frequencies of interest.

But, it is not easy to design a stethoscope which has a good output at the low frequencies and which can also operate satisfactorily at higher frequencies. It is still more difficult to design a stethoscope which has these characteristics and which in addition has means for emphasizing the frequencies of interest. To solve this problem electrical stethoscopes have been developed which were provided with frequency tone controls for attenuating the response in either the high or low frequency range. This enabled the physician to more clearly near the particular sounds of interest.

However, the compliance of the transducers used with prior electric stethoscopes was fixed and could not be varied regardless of the frequency of the sound the physician was interested in. As a result the resonant frequency of these transducers, and hence their region of greatest sensitivity, was either completely outside the frequency range of interest to the physician or else the resonant frequency and sensitivity of the stethoscope was satisfactory over only a limited frequency range. Consequently, considerable electronic distortionless amplification was necessary over the frequency range of interest. However, this could not be easily or economically achieved. In addition, the requirement for a substantially linear or distortionless amplification made the prior stethoscopes more vulnerable to damage due to impacts.

As will become more apparent below, this invention involves the discovery that efficient transducer following of skin vibrations requires a preloading force which compacts the fatty skin surface and any overlying clothing into a continuum of relatively uniform acoustical impedance from the chest cavity into the transducer. If this is not done, acoustical mismatch from the body into the transducer results in lowered energy available to drive the transducer accompanied by a decreased frequency response.

What is needed, therefore, and comprises an important object of this invention is to provide a stethoscope which has means for compacting the skin-clothing transducer interface into a good acoustical transmission path.

As stated above it is desirable for a stethoscope to have the capacity for a good response and sensitivity over not only the lower frequencies but the higher frequencies as well. The variable reluctance transducer employing both an armature and an electromagnetic coil appears to be suitable for these requirements because the output voltage generated by the variable reluctance transducer, assuming a constant amplitude, in general, increases with the frequency. By using a stethoscope with such a transducer the low frequency response desired can be provided by designing the transducer suspension so it has a high compliance at lower frequencies,

while the inherent characteristics of the variable reluctance transducer increases the energy output at the higher frequen cies. However, it is difficult to design a transducer with high compliant suspension and which at the same time is resistant to impact damage.

What is needed therefore, and comprises another object of this invention, is to provide a stethoscope having a variable reluctance transducer wherein the armature has a high compliant suspension at low frequencies and which at the same time is sturdy and highly resistant to impact damage.

Since the physician in using a stethoscope is interested in listening to sounds at various frequencies, it would be desirable to provide a stethoscope with means for emphasizing the frequencies of interest. If a stethoscope is designed with a variable reluctance transducer, the compliance of the armature suspension can be varied in order to emphasize the frequencies of interest and to provide such a stethoscope constitutes still another object of this invention.

A further object of this invention is to provide an electric stethoscope which has means for eliminating clothing noise or the like.

These and other objects of this invention will become more apparent when understood in the light of the accompanying specification and drawing wherein:

FIG. 1 is an elevational sectional view of the electric stethoscope.

FIG. 2 shows the stethoscope housing connected to a speaker and acoustic tubes.

Referring now to FIG. 1 of the drawing, a stethoscope constructed according to the principles of this invention and in dicated generally by the reference numeral 10 is mounted in a suitable generally cylindrical housing 12. To prevent electromagnetic interference the housing 12 may be formed from a molded plastic impregnated with iron powder; alternatively the housing may be formed from other magnetic dielectric materials such as copper, brass or aluminum. Next a thin layer of iron powder could be bonded to the aluminum layer. With this arrangement the powdered aluminum layer would reflect a substantial portion of any electromagnetic field entering the housing through membrane 14. The remaining portion of the electromagnetic field is absorbed in the layer of iron powder. If desired additional alternate thin layers of aluminum and iron powder could be added to the inner surface of membrane 14 without loss of flexibility to prevent the entry of any interfering electromagnetic field inside the housing 12. As will become apparent below this arrangement in combination with the magnetic dielectric material of the housing 12 substantially eliminates interference due to stray magnetic fields. This membrane is peripherally supported between the rim 16 of housing 12 and a compliant support bearing 18. The support bearing has a double function in that it serves to isolate the stethoscope from vibrations in housing 12 and in addition it spaces the body contact membrane 14 from the remaining parts of the transducer. This eliminates interference due to clothing noise, as will become apparent below.

The compliant support bearing 18 is provided with a support member receiving groove 20 for receiving and retaining a metal support plate 22. The support plate 22 is heavy enough to provide the reaction mass necessary to achieve the desired low frequency response. A variable reluctance transducer 24 including an electromagnetic coil 25 is mounted on support plate 22. If desired the upper part of the electromagnetic coil could be covered by a two layer magnetic shield. The first layer 25 adjacent to the outer membrane 14 is a magnetic reflector formed from copper or brass or the like. The inner layer 27 is a magnetic conductor formed from a ferrous material. If necessary additional two layer shields could be added to control the external magnetic field. A generally ring shaped compliant pressure block 26 is bonded at one face to support plate 22. A contact pressure pickup plate 2% is bonded to the other face of pressure block 26. The mass of pickup plate 28 is selected to cut off the unusable high frequency range in the stethoscope. The diameter of pickup plate 28 is preferably one inch in' order to achieve-maximum sensitivity without sacrificing the ability of the stethoscope to locate the source of the sound. In some circumstances the.

not introduce clothing noise into thestethoscope. With.

clothing noise eliminated the physician will be able to listen to the body sounds without interference.

Screws 30 extend through openings 31in pickup plate28, through openings33 in support plate 22 and on into threaded engagement with openings 35 in armature support .32. As seen, the armature 34 is mounted on support 32 by any suitable means. By this arrangement the contact pressure-pickup plate 28 and the armature disc 34 move togetheras a unit. In addition, adjustment of screws 30 provide a means for adjusting the compliance of pressure block 26' for reasons to become. apparent below. h

The support'plate 22-,- pickup plate 28; screws 30, and support 32 are preferably formed from brass or other conductive material which is not influenced by a magnetic field. This ar-- rangement further reduces objectionable noise inthe stethoscope arising from stray magneticfields'.

As shown in FIG. 1, the screws position the armature :disc

34 on the side of "support plate. 22 opposite from the'pickup.

plate 28. With'this arrangement forceonthe body'contactx.

member 14 caused by pressing the' stethoscope againstthe body of a patient forces the body contact'mernber l4 into con tact with the pickup'plate 28. This in turn transmits the force (and the sound) caused by pressing the'stethoscope against the body of a patient to the armature disc 34'so that the arma ture vibrates-at a frequency related to the frequency of sound from the body. The screws 30 and support 32' hold the'armature disc 34 close enough to the electromagnetic coil 25 so that the armature disc'ismagnetically attracted to the coill. This magnetic force is in opposition to-the. force exerted on the armature disc caused when the stethoscope ispressed against the. body of a patient.

The point of highest sensitivity of the variable reluctancetransducer with a given armature vibration amplitude is the.

point wherein the magnetic. path has the lowest average.

reluctance. As seen in the drawing, the magnetic. force.-

betweentheannature. and the electromagnetic coil" is arranged to add to the skin preloading force requirement described. below. The point of lowest armature. air gap reluctance lies in the region adjacent the magnetic poles. Ac-

jectionable voltage surge. in addition, as will become apparent,.the damping sheet exerts an important influenceon the compliance of the armature 34.

The armature damping sheet 36 formed from acompliant 'nonsetting material .002-.005' inches thick may be perforated with a Swiss cheese hole pattern to achieve the. compliance necessary for the desired frequency response. This damping sheet is bonded only to the armature disc 34. A compliantdamping ring 38 mounted'between the rim' 16 of housing 12 andthe contact membrane 14-further isolates the ,body contact membrane from noise due to vibration in the housing. i

The magnetic. force exerted on the transducer armature by the electromagnetic coil 24 and the compression of the compliant pressure block 26 betweenthe pickup plate: 28 and the support plate 22, plus the compression of the damping sheet 36 between the armature disc 34 and the pole faces of the electromagnetic heavily preloads or biases the transducer armature against the electromagnetv As a result, the armature is not free to vibrate until the bias force isbalanced by pressing the contact membrane 14 of the stethoscope against the fatty skin surfaces of the body with enough force to insure a relatively uniform acoustical impedance.

Such preloading prior to use of the stethoscope assures a greater uniformity of sound transmission (as a function of frequency and output) than would otherwise'be available. Not only is sound quality mademore uniform-by uniform minimal skin preloading, but because the transducer structure can be closely coupled by this mechanical preloading to the body cavities, the body sounds are less altered by the mechanical construction details of the armature structure; Furthermore, since the armature cannot vibrate until the bias force is balanced,v as described above, clothing and handling noise caused by movements of the stethoscope over the clothing surfaces of the body will be eliminated or greatly reduced.

With this arrangement the stethoscope has a generally monolithicconstruction with massive-steel parts mounted on compliant nonsetting parts 18, 26 and 36. This provides the low resonant frequency desired for good sensitivity at low frequencies and at the same time the stethoscope parts are sufficiently dampedso the stethoscope will be rugged and able to withstand impacts without damage.

By constructing the stethoscope this way, when the stethoscope is pressed against the body of a patient with sufficient forcerthe body sounds or vibration produce electrical pulses in transducer 24. In addition, the compliance of the bearing block 18, the pressure block 26 and the damping sheet 36 all change. This affects the resonant frequency of the transducer. Consequently, the resonant frequency of the transducer maybe varied by simply varying the force exerted by the physician when pressing the stethoscope against the body of a patient. This has the affect of widening the frequency response of the stethoscope and the stethoscope will have a good sensitivity over a wide frequency range. However, it is apparent that the maximum sensitivity is achieved when the force exerted on the stethoscope by pressing it against the body of a patient exerts a force on the armature disc 34 which is equal and opposite to the magnetic force exerted by the electromagnetic coil 25'. By designing the parts of the stethoscope so that this occurs at a suitable low frequency, the

stethoscope will have a particularly good response in a region where good sensitivity was heretofore most difficult to achieve.

in summary, a light contact force tends to deemphasize the low frequency response of the stethoscope because of the static magnetic pole force exerted on the armature disc 34 reduces the complianceof the damping sheet 36. In contrast, a higher contact force increases the compliance of the damping sheet 36 and stiffens the pressure bias block 26 thus increasing the low frequency response.

By designing the instrument so the resonant frequency of the transducer at low contact pressure corresponds with that of the. Ford Acoustic Stethoscope, and choosing the resonant frequency at moderate contact pressure to correspond with the desirable resonant frequency of the Bowles Acoustic Stethoscope, Applicantsstethoscope will have the desirable frequency response characteristics of these prior Stethoscopes which most physicians are familiar with.

As shown in FIG. 1, .a support 39 is mounted in housing 12. This support defines a chamber 40 for receiving a miniaturized amplifier and other electrical parts (not shown). The chamber 40 is closed off by a closure 41 screw threaded to housing 12. As shown a push button on-off switch 43 is mounted. in closure 41 to control the power to the amplifier. Alternatively, if desired, this on-off switch 43 could be mounted entirely inside the housing 12 and positioned in such a way that the movement of support plate 22 caused by the pressing of the contact membrane 14 against the body of a patient closes the switch 43 and turns the power on to the ampli tier and keeps it on while the pressure is maintained. The advantage of this arrangement would be to. provide an automatic method of turning the power on without requiring additional hand movement by the operator. An acoustic hose 42 (see FIG. 2) is connected to the side of housing 12. A fine soft wire 44 inside the hose is connected at one end to the amplifier and terminates in a speaker 46. The soft wire 44 carried inside hose 42 may be spaced from the walls of the hose to minimize the affect of noise on the surface of the hose. Short acoustic hoses 48 to 50 lead from the speaker 46 to the ears of the user of the instrument to minimize sound attenuation thereby providing a stethoscope which has frequency discrimination without distortion or attenuation.

To this point the structure described has been used to provide an improved stethoscope. However, the same apparatus could also serve as a sound transmitter or speaker. For example, it is well known that ultrasonic sound has a therapeutic effect in treating certain kinds of injuries. By feeding an appropriate signal into the electromagnetic coil, the armature could be caused to vibrate in such a way as to produce ultrasonic vibrations. These would be transmitted to the contact pressure pickup plate and the body contact membrane. Then when the body contact membrane is pressed against the body of a person with sufficient force to provide a good acoustical path, the ultrasonic sound would be transmitted into the body. Additionally, the same apparatus could be used as an ultrasonic generator for industrial purposes, such as cleaning delicate equipment.

Similarly, the apparatus could also serve as a speaker by feeding an appropriate signal into the electromagnetic coil. in such an operational mode the pressure of the body contact membrane against a body would provide a good acoustical path for sound transmission into the body and could increase the signal to noise ratio if the surrounding area were noisy. In particular if the apparatus were connected to an automobile radio and a body contact membrane was pressed firmly against the body of a person, it is thought that sounds received by the radio could be heard more clearly against a background of wind and motor noise.

Obviously, many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention can be practiced otherwise than is specifically described.

lclaim:

1. In a stethoscope of the class described, a housing, a body contact membrane mounted on said housing for engaging the body of a patient, a support plate in said housing, a transducer mounted on said support plate, said support plate connected to said housing through a compliant bearing to isolate the transducer from housing vibrations, said transducer including a vibratory part, a portion of said vibratory part positioned near said body contact membrane such that only when said body contact membrane is pressed against the body of a patient said body contact membrane engages said portion of said vibratory part and causes said transducer to generate electrical vibrations, compliant means interposed between said vibratory part and said support plate in such a way that when the body contact membrane is pressed against the body of a patient, the compliance of the compliant means varies in accordance with the pressure whereby the resonant frequency of the transducer may be varied so that the stethoscope will be more sensitive in the frequency range of interest.

2. In a stethoscope of the class described, a housing, a body contact membrane mounted on said housing for engaging the body of a patient, a compliant bearing mounted on said housing, a support plate mounted on said compliant bearing, a transducer including an electromagnetic coil mounted on said support plate whereby the transducer is isolated by said compliant bearing from housing vibrations, a compliant pressure block bonded at one surface to said support plate, a contact pressure pickup plate, the surface of said compliant pressure block opposite to said one surface bonded to said contact pressure pickup plate, the thickness of saidcompliant pressure block selected to position said pickup plate close to said body contact membrane such that when pressure is exerted on said body contact surface said body contact membrane is forced into engagement with said pickup plate, an armature associated with said electromagnetic coil, said pickup plate connected to said armature in sucha way that sound detected by said pickup plate causes said pickup plate-and said armature to vibrate, the compliance of said compliant bearing block varying in accordance with the pressure exerted on said pickup plate whereby the resonant frequency of the transducer may be varied so that the stethoscope will be more sensitive in the frequency range of interest.

3. An apparatus of the class described comprising a housing, a body contact membrane mounted on the housing for engaging the body of a patient, a compliant bearing mounted in said housing, a support plate mounted on said compliant bearing, a transducer including an electromagnetic coil and an armature, said electromagnetic coil mounted on said support plate, a compliant pressure block bonded at one surface to said support plate, a contact pressure pickup plate secured to the surface of said compliant pressure block opposite to said bonded surface, the thickness of said pressure block selected to position said pickup plate adjacent to said body contact membrane whereby when pressure is exerted on said body contact membrane, it is forced into contact with said pickup plate, said pickup plate connected to said armature and positioning said armature on the opposite side of said support plate with respect to said pickup plate, whereby the pressure on said body contact membrane is transmitted to said armature opposing the magnetic force acting on said armature, said armature positioned so it is not free to vibrate until the body contact membrane is pressed against the body of a patient with sufficient force to compact the flesh, and/or clothing transducer interface to achieve acoustical matching between the body and the transducer for more efficient reception of sound from the body or transmission of sound to the body, whereby the maximum sensitivity of the apparatus generally coincides with the optimum acoustical matching between the body and the transducer.

4. An apparatus of the class described, comprising a housing, a body contact membrane mounted on the housing for engaging the body of a patient, a compliant bearing mounted in said housing, a support plate mounted on said compliant bearing, a transducer including an electromagnetic coil mounted on said support plate, whereby the transducer is isolated by said compliant hearing from impact damage, a compliant pressure block, one surface of said pressure block secured to said support plate, a contact pressure pickup plate, the opposite surface of said pressure block engaging and secured to said pickup plate, the thickness of said compliant bearing selected to position said pickup plate close to said body contact membrane so that when pressure is exerted on said body contact membrane saidbody contact membrane is forced into the engagement with said pickup plate, an armature associated with said electromagnetic coil, said pickup plate connected to said armature in such a way that vibrations are transmitted between the pickup plate and the armature.

5. An apparatus of the class described comprising a housing, a body contact membrane mounted on the housing for engaging the body of a patient, a support plate mounted on the said body, a transducer mounted on said support plate, said transducer including an electromagnetic coil and an armature, said armature positioned on the opposite side of said support plate with respect tosaid body contact membrane, means connecting said body contact membrane with said armature in such a way that force exerted on said body contact membrane is transmitted to said armature opposing the magnetic force acting on said armature, said armature positioned so it is adjacent the electromagnetic coil whereby the armature is restricted from vibration due to the clamping effect of the magnetic force holding the armature against the coil until the body contact membrane is pressed against the body of a patient whereby the force of said contact transmitted through said connecting means to said armature opposes the magnetic force exerted on said electromagnetic coil permitting the arefficient reception of sound from the body or transmission of sound to the body whereby the maximum sensitivity of the apparatus generally coincides with the optimum acoustical matching between the body and the transducer. 

1. In a stethoscope of the class described, a housing, a body contact membrane mounted on said housing for engaging the body of a patient, a support plate in said housing, a transducer mounted on said support plate, said support plate connected to said housing through a compliant bearing to isolate the transducer from housing vibrations, said transducer including a vibratory part, a portion of said vibratory part positioned near said body contact membrane such that only when said body contact membrane is pressed against the body of a patient said body contact membrane engages said portion of said vibratory part and causes said transducer to generate electrical vibrations, compliant means interposed between said vibratory part and said support plate in such a way that when the body contact membrane is pressed against the body of a patient, the compliance of the compliant means varies in accordance with the pressure whereby the resonant frequency of the transducer may be varied so that the stethoscope will be more sensitive in the frequency range of interest.
 2. In a stethoscope of the class described, a housing, a body contact membrane mounted on said housing for engaging the body of a patient, a compliant bearing mounted on said housing, a support plate mounted on said compliant bearing, a transducer including an electromagnetic coil mounted on said support plate whereby the transducer is isolated by said compliant bearing from housing vibrations, a compliant pressure block bonded at one surface to said support plate, a contact pressure pickup plate, the surface of said compliant pressure block opposite to said one surface bonded to said contact pressure pickup plate, the thickness of said compliant pressure block selected to position said pickup plate close to said body contact membrane such that when pressure is exerted on said body contact surface said body contact membrane is forced into engagement with said pickup plate, an armature associated with said electromagnetic coil, said pickup plate connected to said armature in such a way that sound detected by said pickup plate causes said pickup plate and said armature to vibrate, the compliance of said compliant bearing block varying in accordance with the pressure exerted on said pickup plate whereby the resonant frequency of the transducer may be varied so that the stethoscope will be more sensitive in the frequency range of interest.
 3. An apparatus of the class described comprising a housing, a body contact membrane mounted on the housing for engaging the body of a patient, a compliant bearing mounted in said housing, a support plate mounted on said compliant bearing, a transducer including an electromagnetic coil and an armature, said electromagnetic coil mounted on said support plate, a compliant pressure block bonded at one surface to said support plate, a contact pressure pickup plate secured to the surface of said compliant pressure block opposite to said bonded surface, the thickness of said pressure block selected to position said pickup plate adjacent to said body contact membrane whereby when pressure is exerted on said body contact membrane, it is forced into contact with said pickup plate, said pickup plate connected to said armature and positioning said armature on the opposite side of said support plate with respect to said pickup plate, whereby the pressure on said body contact membrane is transmitted to said armature opposing the magnetic force actiNg on said armature, said armature positioned so it is not free to vibrate until the body contact membrane is pressed against the body of a patient with sufficient force to compact the flesh, and/or clothing transducer interface to achieve acoustical matching between the body and the transducer for more efficient reception of sound from the body or transmission of sound to the body, whereby the maximum sensitivity of the apparatus generally coincides with the optimum acoustical matching between the body and the transducer.
 4. An apparatus of the class described, comprising a housing, a body contact membrane mounted on the housing for engaging the body of a patient, a compliant bearing mounted in said housing, a support plate mounted on said compliant bearing, a transducer including an electromagnetic coil mounted on said support plate, whereby the transducer is isolated by said compliant bearing from impact damage, a compliant pressure block, one surface of said pressure block secured to said support plate, a contact pressure pickup plate, the opposite surface of said pressure block engaging and secured to said pickup plate, the thickness of said compliant bearing selected to position said pickup plate close to said body contact membrane so that when pressure is exerted on said body contact membrane said body contact membrane is forced into the engagement with said pickup plate, an armature associated with said electromagnetic coil, said pickup plate connected to said armature in such a way that vibrations are transmitted between the pickup plate and the armature.
 5. An apparatus of the class described comprising a housing, a body contact membrane mounted on the housing for engaging the body of a patient, a support plate mounted on the said body, a transducer mounted on said support plate, said transducer including an electromagnetic coil and an armature, said armature positioned on the opposite side of said support plate with respect to said body contact membrane, means connecting said body contact membrane with said armature in such a way that force exerted on said body contact membrane is transmitted to said armature opposing the magnetic force acting on said armature, said armature positioned so it is adjacent the electromagnetic coil whereby the armature is restricted from vibration due to the clamping effect of the magnetic force holding the armature against the coil until the body contact membrane is pressed against the body of a patient whereby the force of said contact transmitted through said connecting means to said armature opposes the magnetic force exerted on said electromagnetic coil permitting the armature to vibrate, said connecting means connected to said armature in such a way that said armature is not free to vibrate until the body contact membrane is pressed against the body of a patient with sufficient force to compact the flesh for more efficient reception of sound from the body or transmission of sound to the body whereby the maximum sensitivity of the apparatus generally coincides with the optimum acoustical matching between the body and the transducer. 